Measurements of rotation angle and of linear displacement are widely used in various fields for the control of position, velocity and acceleration. Non-contact sensors used for these purposes are described generally in the first chapter of the Synchro/Resolver Conversion Handbook, Fourth Edition, published by DDC ILC Data Device Corp. (Bohemia, N.Y., 1994), which is incorporated herein by reference.
Commercially-available, non-contact, full-rotation transducers (commonly known as rotation angle encoders) are almost exclusively either optical shaft encoders or electromagnetic resolvers. Both of these types of transducers are well known in the art. They are sold both as integrated devices, which include their own shaft and bearings, and as modular devices, to be mounted on a host shaft.
Optical encoders provide binary level output signals and can be divided into absolute and incremental types. Encoders of the latter type are more popular, due to their flat construction and low cost, despite suffering from the following shortcomings:                Only relative position is measured.        Such encoders are sensitive to mechanical assembly and mounting errors.        The construction of such encoders affords only limited mechanical durability.        
Absolute optical encoders are more expensive, more bulky and usually non-modular. In recent years, a modified absolute encoder was introduced, which provides sinusoidal, rather than binary-level, outputs, which can be interpolated to provide enhanced resolution.
Electromagnetic resolvers, which are described in detail in the above-mentioned Synchro/Resolver Conversion Handbook, are wound inductive components. They are relatively bulky and expensive, but highly durable. Single pole-pair resolvers provide two output voltages, which are proportional to sin θ and to cos θ, wherein θ is the rotation angle. Multi-pole-pair resolvers provide output voltages proportional to sin(nθ) and to cos(nθ), wherein n is the number of pole pairs. The resolution and accuracy of the multi-pole-pair resolver are high, but the output signals do not define the rotation angle unambiguously over a full rotation.
The two-speed resolver is equivalent to a combination of a single pole-pair and a multi-pole-pair resolver on the same shaft. It provides, simultaneously, two pairs of output voltages, which are referred to as coarse and fine channels. By processing both channels, an accurate and unambiguous reading is obtained. This kind of resolver, however, is even more bulky and expensive than its single or multi-pole-pair counterparts.
Linear optical encoders are incremental digital devices which, like incremental rotary encoders, include a reading head which moves relative to a ruler and generates output pulses. Currently, high-accuracy, long-stroke, linear encoders are almost exclusively of the optical type, although there are also some linear encoders based on magnetic principles. There is no capacitive linear encoder that is available commercially as a stand-alone component, but linear capacitive encoders are widely used in digital calipers.
In the context of the present patent application and in the claims, the term “encoder” refers to displacement transducers in which the interaction between the stationary and moving elements is based on a repetitive pattern, with a either binary or continuous output signal. The terms “moving element” and “rotor” are used interchangeably with reference to rotary encoders, as are the terms “stationary element” and “stator.” Likewise, the terms “reading head” and “ruler” refer respectively to the moving and'stationary elements of linear encoders.
Even after many years of development, neither optical shaft encoders nor electromagnetic resolvers provide all of the following desirable features in combination:                Absolute reading with high accuracy and resolution.        Simple construction and low-profile packaging.        Low manufacturing costs.        
Basic Concepts of Capacitive Rotation Angle Encoders
Capacitive, full-rotation, absolute angle encoders (CFRAAEs) convert rotation angle into an output signal based on capacitive interaction between a rotor and a stator. They can be built to emulate the single-pole or multi-pole electromagnetic resolver, i.e., with an output signal that repeats once or more times per rotation, as well as multi-speed resolvers.
CFRAAEs, as described in the patent literature, would be expected to provide significant advantages over optical and inductive encoders. But CFRAAE devices have been entirely absent from the market as the result of a variety of difficulties, not all of which have been fully identified, appreciated, or solved. For example:                Accurate CFRAAE operation demands the discrimination of capacitances under one Femto-Farad (10−15 Farads) in the presence of parasitic capacitances and extraneous interference. Shielding against external interference is therefore of paramount importance.        It has been assumed that CFRAAs requires costly, highly-accurate and stable electronic components. For example, German patent application DE 42 15 702 describes a capacitive angle encoder in which capacitances are individually corrected by laser trimming.        In CFRAAEs described in the patent literature, complex signal conditioning is required. Signal processing systems for use in this context are described, for example in German patent application DE 36 37 529 and in a corresponding U.S. Pat. No. 4,851,835, which is incorporated herein by reference.        There has been a lack of systematic classification and analysis of the various known encoder types. Consequently, novel configurations and possibilities for improvement have not been discovered.        
It has therefore been the prevailing view in the field that a CFRAAE could not be commercially feasible. Only limited-rotation (substantially less than 360° ) capacitive transducers have found practical use, and only in limited applications in which the transducer is integrated in a host system, mainly in optical mirror scanners. Typical limited-rotation transducers are described in U.S. Pat. Nos. 3,312,892, 3,732,553, 3,668,672, 5,099,386 and 4,864,295, which are incorporated herein by reference.
Analog full-rotation transducers, such as electromagnetic resolvers (in contrast with digital, or pulse-counting) transducers, typically provide two orthogonal output signals proportional to the sine and cosine of the rotation angle. Since capacitive coupling, unlike inductive coupling, is always positive, the only way, in general, to obtain a bipolar output in a capacitive transducer is to measure the difference between two displacement-dependent capacitances.
FIG. 1 is a typical schematic circuit diagram illustrating this principle (which is also applicable to capacitive linear displacement transducers). Two complementary excitation voltages Q and Q′ are applied to stationary transmitter plates 41 and 42, respectively. A moving receiver plate 40 is capacitively coupled to both transmitter plates and is connected to a charge amplifier 43, as is known in the art. The output voltage of the charge amplifier 43 is proportional to the difference of the respective capacitances C1 and C2 between receiver plate 40 and transmitter plates 41 and 42. The output of amplifier 43 is processed to provide the amplitude and polarity of the differential capacitance C1–C2, from which the position of plate 40 relative to plates 41 and 42 can be derived.
By analogy with electromagnetic resolvers, CFRAAEs can be made in both single-pole and multi-pole configurations. U.S. Pat. No. 5,598,153, which is incorporated herein by reference, describes a typical single-pole CFRAEE. French patent application 77 29354 describes a multi-pole encoder, in which the overlap between the rotor and a stator varies six times per revolution. The above-mentioned U.S. Pat. No. 4,851,835 describes an encoder in which a single rotor generates both coarse and fine signals.
Various methods are described in the relevant patent literature for converting a variable capacitance into output signal. The methods can be divided into two families:                1. Incorporating the variable capacitance in an oscillator circuit, which responds by varying its frequency, or duty cycle. Such methods are described, for example, in European patent application 0 459 118 A1; in German patent application DE 33 28 421; and in an article entitled “Kapacitives Sensorprinzip zur Absoluten Drehwinkelmessung” (A capacitive sensor principle for absolute angle of rotation measurement), by Arnold and Heddergott, in Elektronpraxis (March, 1989).        2. Incorporating an AC excitation source for obtaining at least one AC or DC output signal which is a function of angle-dependent capacitances in the encoder. Two such outputs are required if a full rotation is to be covered. For example, U.S. Pat. No. 4,092,579, which is incorporated herein by reference, describes a capacitive resolver having one excitation voltage source and two output voltage signals proportional, respectively, to the sine and cosine of the rotation angle. U.S. Pat. No. 4,429,307, also incorporated herein by reference, describes a capacitive encoder with a similar circuit arrangement, except that two excitation voltages of opposite polarities are used.        
Similar approaches are described, for example, in European patent application 0 226 716; in German patent application DE 36 37 529; and in an article entitled “An Accurate Low-Cost Capacitive Absolute Angular-Position Sensor with A Full-Circle Range,” by Xiujun Li, et al., in IEEE Transactions on Instrumentation and Measurement, 45:2 (April, 1996), pp. 516–520.
The accuracy of such CFRAEE schemes based on AC excitation depends on the quality of the excitation voltages. Inaccuracies may result to the extent that the excitation signals are not of high harmonic purity and equal in amplitude, or if there is deviation from exact 90° relative phase shift. The difficulties entailed can be overcome by circuit complexity, as illustrated by FIG. 3 in the above-mentioned German patent application DE 36 37 529. Solutions include complex digital emulation of the analog sinusoidal voltages, as proposed in European patent application 0 226 716, or employing accurate and stable analog circuit elements, as in the above-mentioned article by Li, et al.
German patent application DE 37 11 062 also describes a capacitive position measuring device using AC square wave excitation. The rotation angle is computed based on time sampling of a stepwise signal that results from interaction of the square wave excitation voltages with capacitance that varies with the rotor rotation (as shown in FIG. 2-d of that application). The disadvantage of such discrete sampling is an inferior signal-to-noise ratio (SNR), since sampling of the input voltage ignores its values between sampling times and is prone to noise.